One important point in which these two types of ape differ is in the scope and quality of their voices. The _kulu_ makes a greater range of vocal sounds. Some of them are soft and musical; but those uttered by the _ntyigo_ are fewer in number and harsher in quality. One of these sounds resembles the bark of a dog, and another is a sharp, screaming sound. The _kulu_ evinces a certain sense of gratitude, while the _ntyigo_ appears to be almost devoid of that sentiment. There are many traits in which they differ, but human beings, even within the same family circle, also differ in these qualities. The points in which they coincide are many, and, after a brief review of them, we may consider the question of making two species of them or assigning them to the same. The skeletons--as we have noted--are the same in form, size, and proportion. Their muscular, nervous, and veinous systems are for the most part the same. The character of their food and the mode of eating are the same in each. In captivity they appear to regard each other as one of their own kind; but whether they inter-cross or not remains to be learned. Such is the sum of the likenesses and the differences between the two extreme types of this genus. With so many points in common, and so few in which they differ, it is a matter of serious doubt whether they can be said to constitute two distinct species or only two varieties of a common species. This doubt is further emphasized by the fact that all the way between these two extremes are gradations of intermediate types, so that it is next to impossible to say where one ends and another begins. In view of all these facts, I believe them to be two well-defined varieties of the same species. They are the white man and the negro of a common stock. They are the patrician and the plebeian of one race, or the nobility and the yeomanry of one tribe. They are like different phases of the same moon. The _kulu-kamba_ is simply a high order of chimpanzee. It is quite true that two varieties of one species usually have the same vocal characteristics, and this appears to be the strongest point in favor of assigning them to separate species, but it is not impossible that even this may be waived. Leaving this question for others to decide as they find the evidence to sustain them, we shall for the present regard them as one kind, and consider their physical, social, and mental characteristics. Whether they are all of one species, or divided into many, the same habits, traits, and modes of life prevail throughout the entire group, so that one description will apply to all, so far as we have to deal with them as a whole. Elsewhere will be related certain incidents which apply to individuals of the two kinds mentioned; but in treating of them collectively the term _chimpanzee_ is meant to include the whole group, except where it is otherwise specified. CHAPTER XI Physical Qualities of the Chimpanzee--His Social Habits--Mental Characteristics